Implantable medical devices (IMDs) are devices designed to be implanted into a patient. Some examples of these devices include cardiac rhythm management (CRM) devices. CRM devices include implantable pacemakers, implantable cardioverter defibrillators (ICDs), and devices that include a combination of pacing and defibrillation including cardiac resynchronization therapy. The devices are typically used to treat patients using electrical therapy and to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices can include electrical leads in communication with sense amplifiers to monitor electrical heart activity within a patient, and often include sensors to monitor other internal patient parameters. Other examples of implantable medical devices include implantable insulin pumps or devices implanted to administer drugs to a patient.
Additionally, some IMDs detect events by monitoring electrical heart activity signals. By monitoring cardiac signals, IMDs are able to detect abnormally rapid heart rate, or tachyarrhythmia. Although detecting an occurrence of tachyarrhythmia is important, it can be even more helpful if additional physiologic information is known about the tachyarrhythmia, such as if the tachyarrhythmia is hemodynamically stable or unstable. An IMD that can not only detect tachyarrhythmias, but also discriminate between hemodynamically stable and unstable tachyarrhythmias, can be used to help guide therapy decisions.